The GPS is a satellite based navigation system having multiple orbiting satellites. Theoretically, four or more GPS satellites will be visible from most points on or near the Earth's surface, 24 hours per day. Each GPS satellite transmits two spread spectrum, L-band signals: an L1 signal having a frequency f1 of 1575.42 MHz, and an L2 signal having a frequency f2 of 1227.6 MHz. The L1 signal from each satellite is modulated by two pseudo-random codes, the coarse acquisition (C/A) code and the P-code. The P-code is normally encrypted, with the encrypted version of the P-code referred to as the Y-code. The L2 signal from each satellite is modulated by the Y-code. Hereafter, all references in this document to Y-code apply to P-code as well.
Because the Y-code is much more difficult to acquire directly than is the C/A-code, GPS receivers traditionally acquire the C/A-code first. Then, information useful in acquiring the Y-code signal is obtained and used during Y-code acquisition. However, in some instances, it is desirable to acquire the Y-code without acquiring the C/A-code first.
Direct Y-code acquisition requires an accurate clock reference. Real time GPS clocks or time sources are often driven by quartz crystal oscillator circuits. The oscillator circuit drives a counter which counts oscillator output cycles. In order to minimize power consumption for battery powered applications, it is customary to use a low frequency crystal (for example 32.768 kHz). Most crystals in the 10 kHz to 600 kHz frequency range use a quartz resonating element shaped like a miniature tuning fork. The frequency of tuning fork crystals varies with temperature in an approximately parabolic manner as illustrated in FIG. 2. As a result, the LPTS used in existing GPS receivers have frequency drifts of up to 250 parts per million (ppm) over temperature.
The temperature dependent frequency drifts in the LPTS used in conventional GPS receivers results in very large time errors when the receiver has been off for more than a few minutes. Since Y-code acquisition time is directly proportional to time uncertainty, this in turn results in acquisition performance that is unacceptable for direct Y-code acquisition due to hardware constraints. Consequently, a highly accurate wide temperature range LPTS which can overcome these problems in order to reduce the time required for the GPS receiver to directly acquire the Y-code would be a significant improvement in the art.